In various chemical processes, technologies or related fields, it is necessary to provide a solid body or material with a high effective surface area or a highly active surace. For example, many chemical reactions are catalyzed or promoted by the presence of active sites on a solid substance, merely by reason of the existence of such sites or the presence thereat of catalytically effective substances. Similarly, adsorption in a function of the activity of the surface and, in general, it can be said that fluid interactions are promoted by surfaces having a high activity or by solid materials having an active surface.
Various treatments are known to provide the necessary highly active solid surface and, in general, it is recognized that high-surface activity is a function of a high ratio of effective surface area to volume or weight. A high ratio of effective surface area to weight or volume (high specific area) is generally associated with the fine subdivision of the material on the solid surface and thus many of the activating techniques are designed to deposit or form upon a support resistant to the medium to which it is subjected, a substance with a fine subdivision or in a highly subdivided state.
Thus solids having surfaces which are to be involved in a reaction, either as a reaction promoter or regulator or as a reaction participant, must generally be in a state of fine subdivision if homogeneity is to be insured and, in the case of reaction promoters, a high reaction rate is desired. To avoid the need to continuously mix a finely divided solid with the reactants and thereafter recover the solids from the reaction product (mixing and separation) at a high operating and equipment cost, it is known to precipitate the solids from solutions and to deposit them on supports having large surface areas or to form the solids into porous bodies. The loss in activity which thus results in rendering the active solids more compact is generally tolerable when the advantage of a distributed state of the solid is considered.
Widely differing requirements must be met by active solids as noted above, especially when they are provided in a compact state. They must, for example, be wear-resistant (i.e. resistant to abrasion by fluids and fluid-entrained solids with which they may come into contact), must have a high capacity which does not materially drop over long periods (high useful life), must have a degree of subdivision although compact which will sustain high reaction rates, and must be chemically resistant so that the compact does not deteriorate in the presence of the reaction system. Generally it is not possible to provide a solid system having a high surface area which meets all of these requirements since all or several of them are mutually exclusive.
To produce solid ion-exchange agents, for example, high-molecular-weight microporous bodies are made from low-molecular-weight substances by processes well known in the macromolecular chemical arts, i.e. by polymerization techniques. Any functional groups which may be contained in the starting substances or which may be provided by subsequent treatment deriving from only a limited number of possibilities, e.g. acidic, basic, redox-promoting (reduction-oxidation catalyst) and complexing groups. From this small variety of possible functional groups, it is not possible to provide the full spectrum of properties which may be necessary for a particular active solid. Materials which are deposited on activated carbon and on like porous supports, must adhere firmly and, frequently, a satisfactory bond cannot be obtained because of the nature of the support or the nature of the active materials to be applied thereto. In many cases, a firm bond degenerates with the change in pH or some other properties of the medium with which the solids are contacted. The active materials may thereby be lost in the reaction medium and the solid phase rendered useless.